Basically a few key points:

1) Many people take up cycling to lose weight. Hence they are already fat before they begin
2) Like many have posted, cycling makes you feel very very hungry as it's a calory consuming activity. Most eat more than they consume without realising. A plate of pasta has more calories than a 2 hour ride at moderate pace. The coke you take after your ride has more calories than a 1 1/2 hour ride at slow pace..... you get it.
3) Pro cyclist ride so much than they can't physically eat more than they burn.....a 5 hour ride at slow/ moderate pace burns more calories than you can eat at a buffet....